Generally, the turbine rotor blades often have the blade tips provided with a grouped blades structure in order to suppress generation of vibration or to prevent steam from leaking out of the blade tips during operation.
There is a grouped blades structure called as a tenon-shroud structure. This grouped blades structure, i.e. the tenon-shroud structure, comprises tenons, each of which is respectively provided with tip portion of each turbine rotor blades, and a cover that can be attached to the tenons by caulking or swaging. The tenon-shroud structure combines plural turbine rotor blades as a group by attaching the cover to the tenons.
Thus, the tenon-shroud structure, which is provided with plural turbine rotor blades at the tip portion, combines the plural turbine rotor blades into one group. When providing the tenon with the cover, however, it needs lots of time and effort for caulking or swaging work. In addition, the connected portions do not necessarily have enough strength. There is also another grouped blades structure so-called a snubber cover structure. With the snubber cover structure, each turbine rotor blades is provided with a snubber cover (integral cover) at tip ends integrally thereof. These integrally provided snubber covers of each turbine rotor blades connect all the turbine rotor blades circumferentially as a grouped blades.
In connection with the snubber cover structure, there have been disclosed lots of technologies studying on optimization of the cover shape, a degree of connection between the turbine rotor blade and the cover, a connection position and the like (see, for example, JP-A 10-103003 (hereinafter called Patent Reference 1) and JP-A 2007-154695 (hereinafter called Patent Reference 2)).
FIG. 18 is a plan view of an assembled turbine rotor blades 300 having a snubber cover structure viewed from the cover side, namely from radially outside with respect to the central axis (axial direction) of the turbine rotor.
Patent Reference 1 discloses turbine rotor blades 300 having a snubber cover structure, which are grouped by connecting by a cover, as shown in FIG. 18. The turbine rotor blade 300 having the snubber cover structure has a snubber cover 301, as the integral cover, which is integrally provided with the tip of the turbine rotor blade 300. And, a blade suction side 302 and a blade pressure side 303 of the snubber cover 301 are provided with overhanging portions 304 and 305 respectively toward a circumferential direction Cd with respect to the rotational axis of the turbine rotor. When the turbine rotor blades 300 are in an assembled state, the overhanging portion 304 and the overhanging portion 305 are strongly contacted between the adjacent turbine rotor blades 300 along a cover contact surface 308. Cover contact surface 308 intersects with an axial direction of the turbine rotor Ad that corresponds to a normal direction of cover contact surface 308. A reaction force is generated under the strong contact force, and the reaction force is used as frictional force to control vibration. This grouped blades structure is called a snubber cover structure because it controls vibration by using the reaction force as the frictional force.
According to the snubber cover structure, even when heat elongation due to thermal expansion or centrifugal force during the operation is generated in the radial direction, or the pitch of the adjacent snubber cover 301 tends to open by a difference in thermal expansion between the turbine wheel and the snubber cover 301, a positional relationship (interplanar distance) of the individual snubber covers 301 is not substantially affected because a frictional force acts on the cover contact surfaces 308 between the adjacent turbine rotor blades 300. Therefore, the turbine rotor blades 300 having the snubber cover structure can be applied to any turbine stages without limitations, regardless of the turbine rotor blades 300 having, for example, a variable blade length, a temperature difference, a difference in linear expansion coefficient among materials and the like. Patent Reference 2 discloses a turbine rotor blade that can control vibrations by assuring a contact reaction force between the snubber covers. FIG. 19 is a side view of a turbine rotor blade 310 having a twist lock structure.
The turbine rotor blade 310, as shown in FIG. 19, has a twist lock piece 312 (a protruded portion) formed on a bucket dovetail 311 of the turbine rotor blade 310. Bucket dovetail 311 is a portion for implanting the turbine rotor blade 310 to a rotor dovetail provided with the turbine rotor (i.e. turbine wheel 315). A turbine wheel 315, in which the turbine rotor blades 310 are implanted is formed with a twist-return restraint piece 316 (a cutout groove). Twist lock piece 312 is fitted to twist-return restraint piece 316.
The twist lock structure, comprising twist lock piece 312 and twist-return restraint piece 316, enables to stably and surely secure the contact reaction force of the cover contact surface of the snubber cover structure. Since the twist lock structure surely prevents the snubber covers from making a twist return during operation, the circumferentially grouped structure of the turbine rotor blades can be secured.